1. Field of the Invention
The present invention relates to the field of thermal machines and components which are subjected to high thermal stress in use and are provided with a heat insulation layer or a metallic protective layer. It refers, in particular, to a method for the repair of damaged places on these layers.
2. Brief Description of the Related Art
Components subjected to high thermal stress, such as are used, for example, in the blading, the lining of the combustion chamber, or as protective shields in the hot-gas duct of a gas turbine, are often covered with a metallic protective layer or with a multilayer heat insulation layer, in order to protect the basic material lying underneath it against the high hot-gas temperatures. The multilayer heat insulation layer in this case includes a bonding layer (bond coating BC) applied to the basic material and the actual heat insulation layer (thermal barrier coating TBC) which mostly consists of a ceramic material. During operation, a thermally grown oxide layer (thermally grown oxide TGO) also forms at the boundary between the bonding layer and the heat insulation layer and protects the bonding layer against further oxidation and corrosion and further improves the bonding of the heat insulation layer for a specific lifetime range.
Owing to the constant alternating thermal load and influence of the flowing hot gases and of foreign bodies entrained in the hot-gas stream, it may happen that, during operation over a lengthy period of time, there are local peelings (and consumption, for example, due to erosion) of the protective coating which then have to be rectified as quickly and as reliably as possible, so that operation can be resumed as quickly as possible and maintained, undisturbed, for as long as possible. For rectification, the sequence of layers of the protective coating has to be built up again in succession in the regions of the local damage, so that the component is fully protected again.
It is also conceivable, however, that, on a component which is otherwise provided with a protective coating, there are from the outset untreated places, for example weld seams or the like, which are free of protective coating and which subsequently have to be provided locally with a protective coating in the form of a metallic protective layer or of a ceramic heat insulation layer.
A method for rectifying a metallic protective layer has already been described in the publication U.S. Pat. No. 6,569,492. EP-B1-0 808 913 discloses a method for rectifying a ceramic heat insulation layer.
Further rectification methods are known from the publications U.S. Pat. Nos. 5,735,448, 6,042,880, 6,203,847, 6,235,352, 6,274,193, 6,305,077, 6,465,040, 6,605,364, EP1304446A1 and U.S. Pat. No. 5,972,424.
In the known rectification methods for protective coatings, the following problems arise:                It is in the nature of metallic protective layers or PC/TBC multilayer systems that the edges of the damaged or peeled-off places have a random configuration without a specific form. There has hitherto been no proposal for classifying the damage as a precondition for a decision on repairability and the use of a corresponding standardized preparation of the damaged place.        Regions which have been predamaged during operation in the metallic protective layer or the BC/TBC multilayer system, but do not appear visibly, cannot be detected in the known methods and therefore also cannot be repaired. This results in a high risk of failure of the component, even if the coating has been rectified locally. So that a full lifetime cycle can be ensured, the entire coated surface or, in particular, the regions put at risk, that is to say regions subjected to particularly high thermal mechanical load, must be examined for mechanical integrity by means of a suitable nondestructive test method.        Since the edge regions of the damaged coating surfaces are irregular, they may be very steep and not have a sufficient slope between the basic material, the BC layer, and the TBC layer. If special precautions are not taken, this may result in uncontrolled preparation during cleaning (including the risk of damaging the contiguous intact coating surfaces), and an overlap effect may occur during the subsequent recoating. This may lead to mismatches in the BC/TBC multilayer system. Components repaired in this way are exposed to a high risk of local peeling on account of a local mismatching of the coefficients of thermal expansion under thermal alternating load. According to the known rectification methods, the local repair of protective coatings is carried out outside the thermal machine. This requires the demounting and transport of the components to be repaired and leads to losses of time and increased costs.        